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I wrote:
>
> When the depth of the corrosion is <10% of the material's thickness,
> you then estimate the AREA of the corrosion. If it covers more than
> 20% of the surface area, the part is usually tagged for replacement.
>
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Which caused someone to ask,
"Can you expand on that?"
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Mebbe. It's really the inspector's call, not a rigid rule.
Where the corrosion occurs is the clue. With Alclad you can expect to
see it on edges, that feathery looking discoloration that may not be
significant. But could be.
You also tend to see it around rivets, where it may brush away, giving
the impression it is not a problem.
If it's fairly extensive, such as on every edge or around every rivet,
even if it looks to be only minor surface corrosion the inspector will
ask (or, SHOULD ask...) to see what's UNDER those edges, requiring you
to drill out a few rivets, separate the parts and take a peek at what's
underneath.
Sometimes there's no problem. Sometimes. But the edge-corrosion may
have etched a line in the lower part creating an unacceptable
stress-riser. Or the edge-corrosion my have bled back farther UNDER
the piece than on the exposed surface.
Drilling out a rivet, you sometimes find the edges of the hole has
turned into a powdery circle (lots of riveters store the rivets in
their MOUTH, treating each rivet and the hole it goes into to a free
dose of spit). A corroded rivet hole may clean-up by going up HALF a
size, in which case you may be required to dismantle the thing,
re-drill and re-assemble with new, plus-size rivets.
Lotsa stuff like that.
Corrosion begs the question: "How bad is it?"
If you treat only the VISIBLE corrosion you leave the question
unanswered. The inspector's job is to come up with a definitive answer,
which then dictates whether the part can be repaired or if it needs to
be replaced.
Okay, so it's a royal pain in the ass and the part is probably good for
another twenty years. Try looking at it this way: You're safe, feet
firmly on the ground, probably warm & dry over in the corner of some
hangar. So you take the time to find the answer; to leave nothing to
Chance. Do the job right, it's a life-time sort of thing. And yes,
there's several ways to interpret that :-)
-R.S.Hoover
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Re: Lye as an aluminum cleaner?
